From the Super Mario Wiki, the Mario encyclopedia
The Fine Mushroom is a recovery item. Upon being used, it will remove all bad conditions from the user's kart, such as square tires, burst tires, or Thunder Cloud. In addition, the Fine Mushroom provides a shield for approximately half a second upon being used. With good timing, this can be used to block incoming items or be immune to the explosion of a Time Bomb.
In Mario Kart Arcade GP, a variant of the Fine Mushroom known as the Fine Mushroom+ can be unlocked. This variant greatly increases the duration of the shield the item provides.